1. Field of the Invention
The present invention relates to a pinned layer structure for a spin valve sensor having cobalt iron (CoFe) and cobalt iron oxide (CoFeO) laminated layers and, more particularly, to such a pinned layer structure wherein the cobalt iron oxide (CoFeO) layers cause specular reflection of conduction electrons and each cobalt iron oxide (CoFeO) layer has a thickness within a range which permits the cobalt iron (CoFe) layers to be exchange coupled with respect to one another.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with respect to the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is at a minimum and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering in response to signal fields from a rotating disk changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layers and R is the resistance of the sensor when the moments are parallel. A sense current IS is conducted through the spin valve sensor by processing circuitry so that when signal fields from a magnetic medium, such as a rotating magnetic disk, cause the magnetic moment of the free layer to move up and down from its parallel position with respect to the ABS, resistance changes of the spin valve sensor are processed as playback signals by the processing circuitry.
In some spin valve sensors an antiparallel (AP) pinned layer structure is substituted for a single layer pinned layer structure. The AP pinned layer structure includes a nonmagnetic AP coupling layer between first and second AP pinned layers. The first AP pinned layer is exchange coupled to the antiferromagnetic pinning layer which pins the magnetic moment of the first AP pinned layer in the same direction as the magnetic spins of the pinning layer. By exchange coupling between the first and second AP pinned layers the magnetic moment of the second AP pinned layer is pinned antiparallel to the magnetic moment of the first AP pinned layer. An advantage of the AP pinned layer structure is that demagnetization fields of the first and second AP pinned layers partially counterbalance one another so that a small demagnetization field is exerted on the free layer for improved biasing of the free layer. Further, the first AP pinned layer can be thinner than the single pinned layer which increases an exchange coupling field between the pinning layer and the first AP pinned layer. The AP pinned layer structure is described in U. S. Pat. No. 5,465,185 which is incorporated by reference herein. In addition to a spin valve sensor being either an AP pinned layer structure or a single layer pinned layer structure type of spin valve sensor, it may either be a top or bottom spin valve sensor. In a top spin valve sensor the free layer structure is located closer to the second read gap layer whereas in a bottom spin valve sensor the free layer structure is located closer to the first read gap layer.
The aforementioned magnetoresistive coefficient dr/R of the spin valve sensor is dependent upon the amount of conduction electrons scattered at the interfaces of the spacer layer with each of the pinned layer structure and the free layer structure. Unfortunately, some of the conduction electrons are lost from the mean free path through the pinned layer structure and the free layer structure which reduces the number of conduction electrons in the mean free path and reduces the magnetoresistive coefficient dr/R. There is a strong-felt need to prevent the loss of conduction electrons in the mean free path so as to maximize the magnetoresistive coefficient dr/R.
The present invention provides a pinned layer structure which enhances the mean free path of conduction electrons by specular reflection. Loss of conduction electrons through the pinned layer structure is prevented by making the pinned layer structure a laminate of alternating layers of cobalt iron (CoFe) and cobalt iron oxide (CoFeO). The cobalt iron oxide (CoFeO) layers reflect the conduction electrons back into the mean free path and each cobalt iron oxide (CoFeO) layer is sufficiently thin so as to permit the cobalt iron (CoFe) layers to be exchange coupled with respect to one another. I have discovered this thickness to be within a range from 5-20 xc3x85. The pinned layer structure has a plurality of cobalt iron (CoFe) layers and a plurality of cobalt iron oxide (CoFeO) layers with one of the cobalt iron (CoFe) layers interfacing the pinning layer and another one of the cobalt iron (CoFe) layers interfacing the spacer layer. Specular reflection is fully described in an article entitled xe2x80x9cOxygen as a Surfactant in the Growth of Giant Magnetoresistance Spin Valvesxe2x80x9d found on pages 6142-6151 of J. Appl. Phys. 82 (12). Dec. 15, 1997.
An object of the present invention is to provide a pinned layer structure for a spin valve sensor which will reflect conduction electrons back into a mean free path so as to prevent a loss in a magnetoresisuive coefficient dr/R of the spin valve sensor.
Another object is to accomplish the foregoing object without a loss of magnetic moment of the ferromagnetic layers within the pinned layer structure.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.